Fluorescent properties of marine phytoplankton exudates and lability to marine heterotrophic prokaryotes degradation

Exudates by the diatom Phaeodactylum tricornutum were incubated with a natural community of marine heterotrophic prokaryotes for 24 d in order to investigate the link between the biological lability and the molecular weight, fluorescence, and polarity of phytoplankton dissolved organic matter (DOM). Dissolved organic carbon (DOC) removal, changes in fluorescence and in the heterotrophic prokaryote abundance were followed over time both in the total exudates and in the low- and high-molecular-weight fractions. To detect changes in the polarity of proteins, reverse-phase high-performance liquid chromatography (HPLC) was applied to the high-molecular-weight fraction. Our results indicate that freshly produced phytoplankton DOM exhibits a dynamic pattern of degradation that is accompanied by large changes in the growth efficiency of the bacterial community that are likely related to changes in DOM quality. Approximately 20% of high-molecular-weight DOM and 40% of fluorescence attributed to protein-like DOM were degraded over the first days of the incubation indicating that protein-like DOM is likely a labile component of phytoplankton exudates. In contrast, fluorescence measurements suggest that humic-like substances are resistant to bacterial degradation over the 24 d of the experiment. Despite fluctuations in the short-term rates of high-molecular-weight and low-molecular-weight DOM removal, the relative contributions of these fractions to DOM pool were similar in the fresh exudates and at the end of our incubation experiments.